Someone who resembles a heavier Matt Damon and hits on women in Public.
The only way to defend against a Fat Damon is to address everyone in the vicinity, and in the most retarded voice possible proclaim, "Faaaaat Daaaamon".

Guy1: "Hey buddy, you better keep an eye on your girl. Looks like she's getting Fat Damoned at the bar."

Guy2: Approaches the bar gaining the attention of all around and proclaims in a retarded voice, "Faaaaaat Daaaaamon."